1. Field of the Invention
The present invention relates to a power module that houses highly exothermic electronic components such as a power semiconductor device and is used for power conversion or the like, and a method of manufacturing the power module.
2. Related Background Art
In recent years, electronic equipment has been requested to be capable of high performance and reduced in size. Accordingly, a power supply circuit in the field of power electronics, which is used in the electronic equipment, also has been requested to be downsized and have reduced energy consumption. This has raised an important problem of a heat radiating structure of a power module.
As a method of improving heat radiation of a power module, a heat sink is mounted to an electronic component generating a large amount of heat such as a semiconductor chip so that the heat is radiated from the heat sink. Thermal contact between the heat generating component and the heat sink is maintained through an insulative and thermally conductive member. An example of such a member is thermally conductive grease. However, the thermally conductive grease is not easy to handle, and the degree of thermal contact depends on how the grease is applied. Therefore, a method has been employed recently, in which the heat generating component and the heat sink are brought into contact with each other by being fixed in a state where a thermally conductive elastic sheet is interposed between and pressed by the heat generating component and the heat sink.
As another method of improving heat radiation of a power module, a method has been employed in which a substrate having an excellent thermal conduction property is used, and a heat generating component is mounted on the substrate so that heat is radiated from the substrate. An example of such a substrate for heat radiation is a substrate formed by bonding a copper plate to a surface of a ceramic substrate of, for example, aluminum oxide, aluminum nitride, or the like. However, this substrate has presented a problem of a cost increase. Therefore, in applications requiring relatively low power, generally, a metallic base substrate is used in which a wiring pattern is formed on one face of a heat sink of aluminum or the like through the medium of an insulating layer. However, in the metallic base substrate, in order to enhance thermal conduction, an insulating material is required to be thin. Thus, the metallic base substrate has presented problems of influence of noise exerted between metallic bases and low dielectric strength.
As described above, in the ceramic substrate and the metallic base substrate, it is difficult to achieve both high performance and cost reduction at the same time. As a solution to this, a substrate has been proposed in which a lead frame is formed on a surface of a substrate formed by dispersing an inorganic filler in a thermoplastic resin. However, this substrate is formed in such a manner that the thermoplastic resin and the inorganic filler are melted and kneaded to be molded by injection, and thus it is difficult to attain the high concentration of the inorganic filler, thereby presenting a limit to the enhancement of a thermal conduction property. Furthermore, a method has been proposed (JP10(1998)-173097 A) that employs a substrate on which a lead frame is formed on a surface of a substrate formed by dispersing an inorganic filler in a thermosetting resin composition. This substrate is formed in such a manner that a sheet-like material containing a thermosetting resin and the inorganic filler, which exhibits flexibility in an uncured state, and the lead frame are laminated, and then the sheet-like material is cured. In this method, it is possible to attain the high concentration of the inorganic filler, thereby realizing an excellent thermal conduction property.
These substrates for heat radiation employ one-sided wiring formed on a single layer, and thus it is extremely difficult to form microscopic wiring. In a power module such as an inverter, a power circuit portion including heat generating components is mounted on any of these substrates for heat radiation, and a control circuit portion that is composed of components relatively less exothermic such as a driving circuit and a protective circuit and requires microscopic wiring is provided on a control substrate formed of a printed wiring board. The control substrate is fabricated separately from the substrate for heat radiation on which the power circuit portion is formed and connected electrically to the substrate for heat radiation. For example, on a surface of the substrate for heat radiation, on which the components are mounted, the control substrate is disposed at a predetermined space so as to be opposed to the substrate for heat radiation and fixed for packaging.
A power module using the thermally conductive elastic sheet or the substrate for heat radiation described above has presented the following problems.
The thermally conductive elastic sheet is thin, thereby having the disadvantage of low dielectric strength. Further, when a plurality of the heat generating components and one heat sink are brought into thermal contact with each other through the medium of the elastic sheet, it becomes necessary to allow variations in height of the heat generating components mounted on a substrate ascribable to unevenness in height, dimensional tolerances, and variations in mounting posture with respect to the substrate to be absorbed by deformation of the elastic sheet caused by pressing. The thin elastic sheet has a limit to the capability of absorbing variations in height of the components, and thus when there is a wide range of variations, the degree to which the elastic sheet and the heat generating components are in contact with each other cannot be maintained evenly, which is disadvantageous. Moreover, stress caused by the elastic sheet being pressed against the heat generating components with an excessively large force results in damages to the heat generating components and the occurrence of cracks in the elastic sheet, which also is disadvantageous.
A power module using the substrate for heat radiation has presented the following problem. That is, with respect to a heat generating component requiring heat radiation, when an increased number of connecting terminals are used and a pitch between electrodes is narrow, microscopic wiring is required. However, the substrate for heat radiation employs the one-sided wiring formed on the single layer, thereby making it difficult to achieve high-density mounting. On the other hand, when this heat generating component is mounted on the control substrate that allows microscopic wiring but has a poor thermal conduction property, heat radiation is hindered. Thus, the substrate for heat radiation has the disadvantage of being unsuitable for the heat generating component requiring both microscopic wiring and heat radiation to be achieved.
Furthermore, as methods of mounting a semiconductor chip, generally, a wire bonding method and a flip chip mounting method are known. In the flip chip mounting method, a semiconductor chip is mounted facedown on a substrate so that a surface of the substrate and a surface of the semiconductor chip (an electrode-forming surface) are opposed to each other. The flip chip mounting method allows higher-density mounting than in the wire bonding method. However, in a power module using the substrate for heat radiation, it is necessary to allow heat to be radiated from the substrate, and thus when the flip chip mounting method is employed, heat radiation hardly can be expected to be improved. Therefore, in this case, the semiconductor chip as a heat generating component is mounted on the substrate for heat radiation so that a rear surface of the semiconductor chip (a surface opposite the electrode-forming surface) is brought into contact with the surface of the substrate, thereby allowing heat to be radiated from the substrate. Thus, the wire bonding method is employed in which an electrode of a semiconductor chip and an electrode on the substrate for heat radiation are connected using a thin metallic wire. However, in the wire bonding method, the thin metallic wire has a considerably high conductor resistance compared with an on-state resistance of a semiconductor element, and thus when the semiconductor chip is mounted by the wire bonding method, power loss is increased, and an increased amount of heat is generated. Thus, the power module using the substrate for heat radiation has presented the following problem. Since the highly exothermic semiconductor chip is mounted by the wire bonding method, heat radiation needs to be enhanced further with respect to the substrate.
With the foregoing in mind, it is an object of the present invention to provide a power module that allows both microscopic wiring required for a heat generating component including a large number of connecting terminals and excellent heat radiation and provides suitability for high-density mounting and size reduction, and a method of manufacturing the power module.
In order to achieve the aforementioned object, a power module of a first configuration according to the present invention is a power module in which a heat generating component connected electrically to a wiring substrate is connected to a heat sink through the medium of a thermally conductive and electrically insulating member. The thermally conductive and electrically insulating member is a curable composition containing (A) a thermosetting resin, (B) a thermoplastic resin, (C) a latent curing agent, and (D) an inorganic filler. The thermally conductive and electrically insulating member is bonded to the heat generating component in a complementary state to unevenness in shape and height of the heat generating component, and heat generated from the heat generating component is radiated by means of the heat sink.
A power module of a second configuration according to the present invention includes metallic balls provided on a surface of a semiconductor chip, a wiring substrate provided on the metallic balls, and a heat spreader provided closely on an entire rear surface of the semiconductor chip so that heat is radiated from a side of the heat spreader. An electric current flows in a thickness direction of the semiconductor chip. The power module further includes an extraction electrode for electrically connecting the heat spreader to the wiring substrate. The semiconductor chip, the metallic balls on the surface of the semiconductor chip, and the extraction electrode that are interposed between the wiring substrate and the heat spreader are encapsulated with resin.
A method of manufacturing a power module according to the present invention includes the steps of: mounting electronic components including at least a heat generating component on a wiring substrate; forming a curable composition layer containing (A) a thermosetting resin, (B) a thermoplastic resin, (C) a latent curing agent, and (D) an inorganic filler between a heat sink and the wiring substrate on a side of the heat generating component and pressing at least one of the heat sink and the wiring substrate against the other so that a thermally conductive and electrically insulating member is bonded in such a manner as to be deformed complementarily to unevenness in shape and height of the heat generating component; and forming the thermally conductive and electrically insulating member by allowing the curable composition layer to be cured by heating.